Relative file paths for Blueprint files #2130
Description
Hey, thanks for all the incredible work in bringing gtk to Rust!
As noted in #1950, it's often far cleaner to use file to load a composite template rather than resource.
I exclusively use Blueprint (so happy that it's a feature!), and noticed that using #[template(file = "path.blp")] requires a path relative to the project directory, which makes it clumsier to keep markup files next to rust files (as described in that issue above).
Don't know if it would be possible to make Blueprint files behave consistently with how ui files do. But if so, it would be preferable IMO!
Another argument for this: It would make it easier to port an app from .ui markup to Blueprint.